The present invention relates to hydrogen storage alloys for use in rechargeable electrochemical cells. Hydrogen storage alloys provide improved performance characteristics such as high energy density, longer charge retention, longer cycle life and low temperature operation, when used in rechargeable electrochemical systems.
The use of AB5 type alloys for hydrogen storage electrodes is well known. Rechargeable batteries utilizing AB5 type negative electrodes are described for example in U.S. Pat. Nos. 3,874,928, 4,214,043, 4,107,405, 4,112,199, 4,125,688, 4,214,043, 4,216,274, 4,487,817, 4,605,603, 4,696,873 and 4,699,856.
The use of LaNi.sub.5 alloys, which have a high hydrogen content and high hydrogen storing capacity has been widely reported. However, lanthanum is an expensive element, and commercial, large-scale utilization of such an alloy is not practical.
Efforts have been made to keep the excellent hydrogen storage capacity of the LaNi.sub.5 type alloy yet improve the cost and commercial use of such alloys by substitution of various metals into the alloy. The selection and role of modifiers or substitute metals in the LaNi, type alloys depends on the desired properties for the final cell. For electrochemical applications, all performance attributes such as cycle life, rate of discharge, discharge voltage, polarization, self discharge, low temperature capacity and low temperature voltage must be considered when choosing a substitute metal. The effect of such substitutes on cell pressure must be considered since excessive hydrogen pressure can result in loss of aqueous-based electrolyte material, thereby limiting cell life; or otherwise destroying the cell.
While prior art hydrogen storage alloys frequently incorporate various individual modifiers and combinations of modifiers to enhance their performance characteristics, there is no clear teaching of the role of any individual modifier, the interaction of any modifier with other components of the alloy, or the effects of any modifier on specific operational parameters.
U.S. Pat. No. 4,107,405 describes electrode materials based on lanthanum and nickel having a formula close to LaNi.sub.5 in which one of the components is partially substituted by a metal selected from those in groups Ia, II, III, IV and Va of the periodic table of elements, and other than lanthanides, in an atomic proportion which is not zero, being higher than 0.1 percent and lower than 25 percent with respect to the lanthanum.
U.S. Pat. No. 4,487,817 describes a negative electrode of electrochemically active material which consists of an intermetallic compound having the formula AB.sub.m C.sub.n, in which A consists of mischmetal or at least one element selected from Y, Ti, Hf, Zr, Ca, Th, La and the remaining rare earth metals, B consists of two or more elements selected from Ni, Co, Cu, Fe and Mn, and in which C consists of at least one element selected from Al, Cr and Si.
U.S. Pat. No. 4,925,748 describes a hydrogen absorbing alloy represented by the general formula A.sub.1-x B.sub.x C.sub.y D.sub.z, in which A is selected from the group consisting of La, mixtures of La and rare-earth elements, and mischmetal; B is selected from the group Ti, Zr, Ca, Y, Hf and mixtures thereof, C is selected from the group consisting of Ni, Co, Mn, Al, Fe, Cu, Cr and mixtures thereof and D is selected from the group consisting of V, In, Tl, Ga and mixtures thereof.
U.S. Pat. No. 4,983,474 describes a hydrogen absorbing Ni-based alloy comprising Ti, Zr, Mn, V, Fe, Al and Ni. The patent describes the role of each component in contributing to the properties of the electrochemical cell. The patent discusses the role of manganese in improving hydrogen absorption and corrosion resistance in the particular AB.sub.2 -type system disclosed, and further that the effects of manganese are not fully obtained when its content is below 4% by weight.
U.S. Pat. No. 5,032,475 describes a nickel metal hydride cell having a hydrogen absorbing alloy in the mixture for the negative electrode, represented by the general formula, XY.sub.5-a Z.sub.a ; where X is a rare earth element, Y is Ni and Z is at least an element selected from Co, Mn, Ag, V, Cu and B; namely LaNi.sub.5 (mischmetal) and the like in which Ni is partially replaced by Al, Mn, Fe, Co, Ti, Cu, Zn, Zr, Cr or B.
U.S. Pat. No. 5,034,289 describes the addition of a hydrophobic material to a standard hydrogen absorbing alloy such as MnNi.sub.3.55 Co.sub.0.75 Mn.sub.0.4 Al.sub.10.3.
Although prior art alloys have yielded cells that exhibit a quantitative improvement in one or two performance characteristics at the expense of other performance characteristics, the trade-off results in a compromise in performance thus yielding cells with both good and bad characteristics.
High capacity nickel metal hydride rechargeable cell systems tend to build up heat more rapidly during cycling and dissipate heat more slowly depending on the size of the cells. In addition, certain sizes of these cells may fall to less than 80% of their rated capacity on repeated charge/discharge cycles when overcharged in excess of 15%, even at room temperature. The loss of capacity is believed to result from corrosion or degradation of the AB.sub.5 hydrogen storage alloy comprising the active material in the negative electrode and the electrolyte. The corrosion rate is further accelerated by the high operating temperatures and by high temperatures resulting from repeated charge/discharge cycles. The pulverization of the active material in the negative electrode resulting from successive charging and discharging of the cell exposes fresh alloy surface to the electrolyte, further adding to the corrosion process.